The present invention relates to an improved method for processing solvent soluble solids whereby the solids may be recovered in finely divided, particulate form. More specifically, the present invention is directed towards the preparation of an electrostatographic toner material which is suitable for use in electrostatic and xerographic processes.
It is known that images may be formed and developed on the surfaces of certain photoconductive materials by electrostatic means. The basic xerographic process, as taught by Carlson in U.S. Pat. No. 2,297,691 involves uniformly charging a photoconductive insulating layer followed by exposure of layer to a pattern of light and shadow which dissipates the charge on the portions of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light and shadow image. Alternatively, a latent electrostatic image may be formed on the plate directly by charging said plate in image configuration. This image is rendered visable by depositing on the image bearing layer a finely divided electroscopic developing material called a toner. A toner usually includes a thermoplastic resin and a colorant. The toner material will normally be attracted to those portions of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to paper or other receiving surfaces, and the transferred image may be made permanent by heating using suitable fixing means. The above general process is also described in U.S. Pat. Nos. 2,357,809, 2,891,001 and 3,079,342.
Toner materials are most commonly prepared by forming an intimate mixture of a thermoplastic resin and a colorant material, and thereafter comminuting the mixture using a pulverizer, jet mill, or other device to produce particles having an average particle size within the range of about 1 to 30 microns. Other techniques for forming toner material involve the mixing of a colorant with a dispersion, solution or latex of a resinous material followed by spray drying of the mixture whereby descrete particles are formed. General techniques for preparing toner material are disclosed for example in U.S. Pat. RE25/36 and U.S. Pat. No. 3,502,582.
Other techniques for recovering materials in finely divided form include the precipitation technique and the cryogenic grinding technique. The recovery of solvent soluble solids from a solvent by the precipitation into a non-solvent involves the selection of a non-solvent for the solid and the pouring of a solvent solution of the solid into a non-solvent, normally under agitation, causing precipitation of the solid solute. Such procedures have found use in recovering all types of organic compounds including dyes, pigments, aromatic compounds, polymers and the like. Another technique for the reduction of the particle size of solids such as polymeric materials is accomplished by utilizing cryogenic grinding. This practice involves cooling the material down to a very low temperature with dry ice or liquid nitrogen and grinding it in a high shear grinder. This technique is particularly applicable for reducing the particle size of polymers having low softening points or melting points. Typical examples of these and other techniques for polymer particle size reduction are found in U.S. Pat. Nos. 271,080, 1,201,132, 2,067,971, 2,216,094, 2,879,173, and 3,379,797.
While these techniques have generally proven satisfactory in most applications, they do suffer certain disadvantages. For example ordinary mechanical methods of disintegration may cause degredation of the polymeric material accompanied by a reduction in molecular weight. The heat generated during mechanical treatment even under cryogenic conditions is often sufficient to soften the polymeric material giving rise to clogging of the grinding mechanism and destruction of the particulate character of the processed polymer. It is also difficult to precisely control the particle size of the polymeric material so processed and often subsequent screening operations are required to meet target goals in terms of particle size. With regard to the precipitation technique, target particle size goals are often difficult to reach since the solute may precipitate out of solution in the form of a difficult to recover colloid. Also, precipitated polymers having moderately low softening or melting points may have a tendency to congeal unless extremely low temperatures are maintained in the system.
It is thus most desirable to devise a simplified process for the preparation of finely divided solid materials which offers a more precise control over the particle size of the material processed thereby, and which process avoids many of the disadvantages referred to above.